A handheld data processing device such as a mobile telephone or personal digital assistant (PDA) is commonly designed to enable a user thereof to operate a keypad of the device with a thumb or finger of a hand in which the device is held.
It is well known to provide such a device with a touch-sensitive display screen, on certain portions of which images of keys are displayed, the device being operable to perform a function associated with each key in response to the portion of the display screen on which the image of the key is displayed being touched by the user. To date, however, the device has been designed to be held in one of the user's hands, and the display screen touched by a thumb or finger of the user's other hand, or by a stylus held in the user's other hand, and the images of the keys located on the screen in accordance with the assumption that any portion of the display screen may be touched with the same ease as any other portion.